Merehead Quarry (Torr Works), East Cranmore

Geological Context

Torr Works Quarry is situated in the Mendip Hills of Somerset, England, an area known for its Carboniferous Limestone deposits. The quarry lies on the southern limb of the Beacon Hill Pericline, with limestone beds dipping southwards at angles between 20° and 50°.

The primary rock type at Torr Works is Carboniferous Limestone, formed approximately 350 million years ago. This limestone is typically pale to dark grey and well-bedded. In some areas, a younger, horizontally bedded buff-coloured Jurassic oolitic limestone overlies the Carboniferous Limestone, forming an angular unconformity.

Economic and Historical Significance

Torr Works, formerly Merehead Quarry, has been operational since the 1930s. Key milestones include:

• 1938: Taken over by Limmer and Trinidad
• 1958: Sold to Foster Yeoman
• 1970: Renamed Torr Works and connected to the East Somerset Branch railway
• 2006: Acquired by Aggregate Industries

The quarry is one of Europe's largest, producing about 5.5 million tonnes of limestone aggregate annually. It employs over 100 people and contributes an estimated £15 million to the local economy each year.

Torr Works primarily serves markets in the South and South East of England, including London. About 70% of its production is transported by rail, demonstrating a commitment to reducing environmental impact.

Mineral Paragenesis and Sequencing

Based on the Mindat data, Torr Works Quarry hosts 70 confirmed mineral species. The mineral assemblage suggests a complex paragenetic sequence involving both primary and secondary mineralisation.

Primary Mineralisation:
The primary mineralisation likely involved the deposition of calcite, dolomite, and quartz, which are common in Carboniferous Limestone settings. The presence of pyrite and chalcopyrite indicates periods of sulphide mineralisation.

Secondary Mineralisation and Alteration:
Secondary processes have led to the formation of various lead, copper, and manganese minerals. The presence of cerussite, hydrocerussite, and plumbonacrite suggests alteration of primary lead minerals. Malachite and azurite indicate the alteration of copper-bearing minerals.

The formation of iron and manganese oxides and hydroxides (e.g., goethite, hematite, pyrolusite) likely occurred during later oxidation stages, possibly related to near-surface weathering processes.

Type Locality Minerals

Torr Works Quarry is the type locality for 8 minerals, highlighting its geological significance:

1. Mereheadite: Pb₂O(OH)Cl
2. Parkinsonite: Pb₇MoO₉Cl₂
3. Plumbonacrite: Pb₅O(OH)₂(CO₃)₃
4. Rickturnerite: Pb₇O₄[Mg(OH)₄](OH)Cl₃
5. Rumseyite: Pb₂OClF
6. Somersetite: Pb₈O₂(OH)₂(CO₃)₅
7. Symesite: Pb₁₀(SO₄)O₇Cl₄
8. Yeomanite: Pb₂O(OH)Cl

These type locality minerals are predominantly lead-based compounds, reflecting the unique geochemical environment at Torr Works Quarry. Their discovery and characterisation have contributed significantly to mineralogical knowledge.